1. Field of the Invention
The present invention relates to the booting of microprocessor controlled devices.
2. Related Art
A variety of microprocessor controller “smart” devices now include non-volatile memory. Non volatile flash memory, in particular, is now widely used due to its ability to retain information without power and to be rapidly erased and reprogrammed. One interesting use of flash memory is to store boot code or information that a device uses on startup. The boot code is a portion of firmware code stored on the device. Usage of flash memory to store bootcode is advantageous because the firmware, including the bootcode can easily be modified and updated.
Upon booting or startup, a microprocessor reads the code in a specified location of a storage device. Typical microprocessors are generally configured to access and execute code in linear storage devices. The data in linear storage devices is accessed by reading a location specified by, speaking in general terms, a linear address consisting of the row and column of the data. Each memory cell, byte, or bit of data is accessed by specifying its is row and column. The processor will sequentially specify linear addresses from which to read.
In a typical memory system, the protocol to transfer data from the memory to the host is as follows: 1) select the memory device by asserting the chip select line; 2) select the address from which to read by asserting the address of the address bus; 3) assert the read signal. The memory device will respond with the data asserted on the data bus.
A typical program contains instruction data that are stored in various different areas of the memory that are not contiguous or adjacent. Thus, in executing a program, the processor may first execute an instruction from an address in one area and then execute an address from a second (and third etc . . . ) distant or non adjacent area. Furthermore, there is no standard dictating a logical order in which the areas are read or executed from. Each program may execute from different areas according to its own particular routines.
However, some types of storage devices such as flash memory, specifically NAND and AND type flash memory, are not linearly addressable. This means that the processor cannot read or execute code from them upon bootup. The storage space in NAND memory is broken up into discrete groups of data referred to as pages. In order to retrieve the data, the page must first be specified, then the location of the data on the page, specified as an offset from the beginning of the page, must also be specified. However, unlike in linear storage devices, if a page is, for example, 528 bytes in length, byte number 255 cannot be read without first reading the preceding 254 bytes. Furthermore, reading just one byte is a relatively more complicated procedure that does not follow the typical timing requirements of linear memory.